While I don't pratice in your state, here is my take on your question.
First, you didn't give us enough information to give you an accurate answer. Did your Dad have a Will or not? It will make a difference.
If he had a Will, you must get it filed with the Surrogates or Probate court so that an Executor or Executrix can be appointed. It is then their job to accumulate all of his assets, declare them to the court and pay any taxes due on the estate and distribute those assets as the Will dictates.
If he had no Will, its pretty much the same except the Executor is called an Administrator or Administratrix and they do pretty much the same thing as explained above.
If your step mother won't do anything, then you have the right to file a Petition in the Court seeking to be appointed the Administrator of his estate. Since she will have to be served, that should get her moving one way or the other. If it goes as an Administration, then the state determines who gets what according to law and you, as the son, would be entitled to a portion of the estate, the size of which will depend upon how many other siblings exist.
Hire a good estate attorney and start your action.
A clear answer cannot be given on the information presented. Why do you think you are entitled to inherit? Is there a Trust or a Will in place? Has a probate estate been opened?
It is possible your father structured his assets (intentionally or accidentally) in such a way as to leave you nothing.
You likely will need an attorney to help answer these questions and to help you get what you are entitled to if the person handling your father's affairs is not forthcoming with answers, documents or other information.